An energy absorbing steering column mounted on a vehicle, which has characteristics to absorb energy applied to the steering wheel, is well known as a means to alleviate an impact applied to the steering column. Among a variety of structures adopted in the energy absorbing steering column, for example, a ball type where a ball is provided between an outer tube and an inner tube of a steering column, a mesh type where a metallic mesh structure is used in the steering column, a silicon rubber type where silicon rubber sealed in a lower tube is ejected from a slit when an upper tube moves into the lower tube, and the like are used.
According to JP2007-168569A (hereinafter referred to as Reference 1), the energy absorbing steering shaft includes a metallic elastic bush provided between the first cylindrical member and the second cylindrical member for applying a radial elastic force for biasing a first press-fit portion and a first supporting portion in the radial direction, thereby supporting the steering shaft to restrict an axial movement of the first cylindrical member relative to the second cylindrical member. Further, the first cylindrical member includes a second press-fit portion and a slit formed near a front open end of the first cylindrical member in a front-rear direction of a vehicle by cutting a predetermined area in an axial direction, thereby supporting the steering shaft under a condition that the axial movement of the first cylindrical member relative to the second cylindrical member is restricted. Consequently, a load for starting a movement, at which a relative axial movement of the first cylindrical member and the second cylindrical member starts, is set easily and appropriately.
According to the steering column disclosed in the Reference 1, the load for starting a movement is set easily and appropriately. The steering column is structured so that the radial movement of the first cylindrical member relative to the second cylindrical member is restricted by way of a biasing force applied by the metallic elastic bush provided between the first press-fit portion of the first cylindrical member and the first supporting portion of the second cylindrical member near a rear open end in the front-rear direction of the vehicle, and by way of a biasing force between the second press-fit portion of the first cylindrical member having the slit and a second supporting portion of the second cylindrical portion near the front open end. However, when the metallic elastic bush is provided near the rear open end, a large and repetitive load is applied to the metallic elastic bush compared to when the metallic elastic bush is provided near the front open end, which requires a metallic elastic bush made of robust material to withstand the large and repetitive load, invariably resulting in a high cost. Further, a flat portion needs to be formed on an inner tube, and burrs develop during a conventional cutting work for forming the flat portion, which needs to be removed. Still further, a cylindrical blank material having enough thickness needs to be used for assuring necessary rigidity, considering that the cylindrical blank material will be thinned when performing the cutting work, which causes a waste.
A need thus exists for an energy absorbing steering column which is not susceptible to the drawback mentioned above.